Novel electrolysis method

ABSTRACT

A method of electrolysis using an anode comprising an electrically conductive, particularly titanium, substrate at least partially covered with a solid solution-type coating consisting essentially of titanium, ruthenium and tin dioxides.

United States Patent- [1 1 'OLear y *Dec. 17, 1974 NOVEL ELECTROLYSIS METHOD I [56] References Cited [75] Inventor: Kevin J. OLeary, Cleveland UNITED TE PATENT Heights, Ohio 3,491,014 1 1970 Bianchi et al 204/242 I Assigneez Electronor Corporation, Panama 3,701,724 10/1972 Entvisle et al A. 20.4/290 F City, Panama FOREIGN PATENTS OR APPLICATIONS Notice: T portion of the term of this patent 725,492 6/1969 Belgium 204/290 F subsequent to Dec. 4, 1990 has been dis- |ai d Primary Examinen-F. C. Edmundson Filed: Sept. 1973 Attorney, Agent, or Fzrm Hammond & Littell 21 Appl. No.; 400,707 57 ABSTRACT Related US. Application D ta A method of electrolysisusing an anode comprising an 7 [63] continuatiomimpafl of Sen Nd 257,7, May 30 electrically conductive, particularly titanium, substrate 1972, p 3,776,834, which' is a at least partially covered with a solid solution-type continuation-in-part of Ser. 'No. 104,740, Jan. 7, coating consisting essentially of titanium, ruthenium 1971, abandoned. and tin dioxides.

521 US. Cl. 204/123 3 Drawmgs [51] Int. Cl C0lb 7/06, BOlk 3/06 [58] Field of Search 204/95, 128, 290 F 1 NOVEL ELECTROLYSlS-METHOD PRIOR APPLICATIONS This application is a continuation-in-part application of my copending US. patent application Ser. No. 257,717 filed May 30, 1972, 'now US. Pat. No. 3,776,834 which in turn is a continuation-in-part of application Ser. No. 104,740 filed January 7, 1971, now abandoned.

I STATE OF THE ART Recent years have seen a rise in the'popularity of dimensionally stable electrodes, particularly dimensionally stable anodes, especially in those areas where consumable electrodes, such as graphite anodes, had previously been almost exclusively'employed. The technology' most affected to date has been the chlor-alkali industry wherein these electrodes have made remarkable inroads into an area traditionally dominated by graphite anodes. r

While differing significantly in their properties, the various dimensionally stable anodes proposed and in use generally employ some combination of a valve metal and a precious metal. In the initial stages of de-.

velopment, the electrodes proposed were valve metals provided with a precious metal coating, e.g platinized titanium. Despite extensive efforts throughout the chlor-alkali industry, however, such anodes found little practical application, primarily owing to the high rate of consumptionof precious metal during production. Claims of the literature to the contrary notwithstanding,in factit-was found difficult to reduce the wear rate of these anodes to a value of less than 0.5 gram of precious metal per -ton of chlorine produced. When one weighs the price of precious metals against the fact that in 1969 in the United States alone. greater than 9,400,000 tons of chlorine were produced, it becomes apparent that such a wear rate is indeed prohibitive.

A significant advance in the art was the development of the mixed oxide, or solid solution-type, electrode coating. Commonly, an electrode again comprises a valve metal base, generally titanium, however the coating is a solid solution of a valve metal oxide and a precious metal oxide, typically, titanium dioxideruthenium dioxide. In these solid solution coatings, atoms of valve metal in the characteristic rutile valve metal oxide crystal lattice are randomly replaced with atoms of precious metal. Theresult is an electrode which, when employed as an anode in the electrolysis of an aqueous sodium chloride solution, for example,

while allowing a reduction in the amount of ruthenium employed.

It is another object of the invention to electrolyze an aqueous solution containing chloride ions.

- 1 These and other objects and advantages of the invention will become apparenttoj those skilled in the art from the following detailed description.

THE INVENTION The novel electrolysis of the invention comprises passing an electric current through an aqueous electrol yte'containing chloride ions between an anode and a cathode whereby chlorine gas is formed at the anode.

and the cation is reacted at the cathode, the anode comprising an electrically conductive substrate bearing on at least a portion of the surface thereof a solid solution-type coating consisting essentially of:

.may bev any of the types of solutions that have been pre viously electrolyzed. Forexarnple, sodium chloride solutions or sea water may be electrolyzed to form chlorine or hypochlorite which have industrial uses or for electrowinning of metals such as nickel from hydrochloric acid solutions using as the cathode the metal to be electro deposited.

In essence the discovery is that from 35 to mole percent of the ruthenium dioxide present in the coating may be replacedwith tin dioxide. Expressed in another manner, from about 16 to 24 weight percent of the total coating is SnO For example, and according to the preferred embodiment, in place of a coating containing 2 moles of TiO per mole of RuO may be used a coating, containing, for every 2 moles of TiO 0.5 mole of R1102 and 0.5 mole of.SnO.'The potential of suchan anode, as measured for example by its chlorine overvoltage (that is, the potential at which chlorine is discharged when the electrode is employed as an anode in an aqueous sodium chlorine solution), is at least equal to a tinfree anode, with tin-substituted anodes overvoltage often, in fact, being somewhat lower. F urnot only exhibits a low chlorine overvoltage but further OBJECTS OF THE INVENTION Y It is an objectof the present invention to provide a method of electrolysis using a dimensionally stable anode exhibiting at least the advantages of a titanium dioxide-ruthenium dioxide solid solution electrode,

ther, the spread between the potentials at which chlorine and oxygen are discharged at an anode containing SnO within the stated range, is further displaced than when employing aconventional TiO /RuO anode. The result of this is less contamination of chlorine gas with oxygen and a higher Cl efficiency.

In addition to the foregoing and its readily apparent implication'of savings in raw materials based on the dif' ference in price between tin and ruthenium, a further, significant and unexpected advantage accrues. When a titanium anode coated with a TiO -RuO solid solution in which 50% of the RuO has been replaced with SnO (2TiO :0.5RuO 0.5SnO is employed as an anode for chlorine production, a wear-rate of only 0.01 gram of ruthenium per ton of chlorine is evidenced. Compared with a non-tin substituted anode of the same nature (2TiO zRuO this is a difference of one order of magnitude (0.01 vs. 0.l0 0.'l5).

The implications of the foregoing findings will be readily apparent to one skilled in the art. To begin with,

a reduced amount of ruthenium may be employed in lowered. However, perhaps most significantly, the life expectancy of an anode in a commercial installation is extended. That is, for every one year of life of a conventional TiO RuO .solid solution-coated titanium anode in a chlorine cell, an active life of 1.4 to 3 years will be obtained by substituting 50 percent of the RuO with SnO By use of the term electrically conductive substrate it is intended to refer to any material which exhibits sufficient mechanical strength and chemical resistivity to support the coating in the intended application. Generally valve metals, such as titanium, tantalum, zirconium and niobium are used owing to their relatively good conductivity and especially to their ability to form an inert oxide coating under anodic conditions. If desired, the substrate may have a core of a more conductive material, such as copper. Another variation is the provision of special intermediate" layers on the substrate, such as oxygen barrier layer, e.g., manganese dioxide.

Solid solutions of the general type improved upon by the present invention are disclosed for example in US.

Pat. No. 3,632,498 to H. Beer. The particular solid solution disclosed in Beers patent with which the invention is concerned is the titanium dioxide: ruthenium dioxide coating. Methods disclosed in the aforementioned patent which results in a solid solution of this type are useful according to the practice of the present invention. Especially useful are the thermochemical techniques whereby successive layers of solid solution are deposited with intermediate heating in air as is described more fully in the specific example hereinbelow.

The mole ratio of titanium dioxide: ruthenium plus tin dioxide is within the range of l.52.5:1. Within this range then, from 35 to 50 mole percent of the Ru may be replaced with SnO Withamounts greater than 50 percent of SnO the potential of the resultant electrode will be generally prohibitively high. Conversely, with amounts less than 35 mole percent of SnO the only significant advantage is the elimination of a minor portion of more costly ruthenium, advantages such as the oxygen-chlorine potential displacement not being realized to a significant extent. Preferably, as the ratio of TiO :RuO Sn0 increases, the amount of tin substitution will decrease somewhat.

As with other dimensionally stable electrodes, only a portion of the electrically conductive substrate need bear the electrically conductive coating, although the coating may of course be continuous. While the amount of coating applied per square foot of anode surface will vary according to considerations known to those skilled in the art, amounts within the range of from 3 to 6 grams per square foot (total oxides) may be conveniently applied for most commercial purposes. Porous protective coating, e.g., ceramics, may be ap plied over the tin-substituted coating if desired.

It will be understood that the invention is independent of the mechanical configuration of the substrate and hence may take any shape which will allow the application of the coating. Thus, the electrodes may take the form of a wire, rod, cylinder sheet or the like. Further, if the electrode is present in a sheet or plate form, it may be either solid or foraminous. Other configuration most useful in a particular application will be apparent.

In order that those skilled in the art may more readily understand the present invention, the following specific examples are afforded.

EXAMPLE A master coating solution is as follows:

RuCl .2.5H O (38.6% Ru) The solution was prepared by partially dissolving the tin and ruthenium salts in the HCl and adding the butanol. After stirring until the salts dissolve, the butyl titanate was added and the solution was again stirred to ensure complete intermixing. Analysis of the solution gives the following composition in grams per liter: Ru 25.5, Ti 44.3 and Sn 20. This represented a TiO (RuO SnO mole ratio of 2.221 and 40.0 mole percent of smo in (RuO SnO A portion of this master solution was applied to an expanded titanium mesh substrate and the thus-coated titanium was heated in air to a temperature of 450C for 7 minutes. This procedure was repeated 10 more times to result in a final coating weight of 1.5 grams per square foot of anode surface on a (RuO SnO basis.

Anodes prepared according to the foregoing were installed in a conventional mercury cell for the production of chlorine and caustic. After approximately 8 months operation, during which time the cell in question produced 19.4 tons of chlorine, the anodes were removed and analyzed to determine the amount of ruthenium remaining. The average wear-rate of the an odes was determined to be 001 gram of ruthenium per ton of chlorine produced. Anodes operating in the same cell room, differing only in that no tin was present in the coating (a mole ratio of 20:1 still being used), showed an average wear-rate of 0.10 gram per ton of chlorine. In addition, the cells employing the tinsubstituted anodesexhibited average potentials 0.04 volt lower than the potential of cells employing the conventional DSA.

In another test of an electrode prepared as above, it was employed as the anode opposite an asbestoscoated steel mesh cathode in a diaphragm cell. Analysis of the cell gases indicated that between 22 and 25% less oxygen was produced than when employing the usual TiO :RuO on Ti anode.

While the invention has been described with reference to certain preferred embodiments thereof, it is not to be so limited as is clear from the specification and appended claims.

I claim:

l. A method of electrolysis comprising passing an electric current through an aqueous electrolyte containing chloride ions between an anode and a cathode whereby chlorine gas is formed at the anode and the cation is reacted at the cathode, the anode comprising an electrically conductive substrate bearing on at least a portion of the surface thereof a solid solution-type coating consisting essentially of:

a. titanium dioxide,

b. ruthenium dioxide and 2. The method of claim 1 wherein the anode substrate is titanium.

3. The method of claim 2 wherein the coating contains 2 moles of tin dioxide for each mole of combined ruthenium and tin dioxides and the tin dioxide represents about 45 mole percent of the combined ruthenium and tin dioxides. 

1. A METHOD OF ELECTROLYSIS COMPRISING PASSING AN ELECTRIC CURRENT THROUGH AN AQUEOUS ELECTROLYTE CONTAINING CHLORIDE IONS BETWEEN AN ANODE AND THE CATION WHEREBY CHLORINE GAS IS FORMED AT THE ANODE AND THE CATION IS REACTED AT THE CATHODE, THE ANODE COMPRISING AN ELECTRICALLY CONDUCTIVE SUBSTRATE BEARING ON AT LEAST A PORTION OF THE SURFACE THEREOF A SOLID SOLUTION-TYPE COATING CONSISTING ESSENTIALLY OF: A. TITANIUM DIOXIDE, B. RUTHENIUM DIOXIDE AND C. TIN DIOXIDE, THE MOLE RATIO OF TIO2:RUO2+SNO2 BEING WITHIN THE RANGE OF 1.5 TO 2.5:1 AND THE TIN DIOXIDE REPRESENTING FROM 35 TO 50 MOLE PERCENT OF THE COMBINED RUTHEINIUM AND TIN DIOXIDE.
 2. The method of claim 1 wherein the anode substrate is titanium.
 3. The method of claim 2 wherein the coating contains 2 moles of tin dioxide for each mole of combined ruthenium and tin dioxides and the tin dioxide represents about 45 mole percent of the combined ruthenium and tin dioxides. 